1. Field of the Invention
The present invention relates to a direct digital control system and more particularly to an improved digital controller system permitting both a sweeping reduction in wiring and the addition of new control loops without the addition of the process input-output devices required in conventional digit controller system.
2. Description of the Prior Art
Generally, in a well known DDC (Direct Digital Controller) system connected to a plurality of control systems, a variable measured value to be controlled is transmitted from a detector of each of the control systems and is compared with a preset reference value by the digital computer. A control signal generated as a result of a control operation by the computer is sent back to an operating apparatus of the control system and the control system reduces the error or the deviation between the measured value and the reference value to zero by driving the apparatus.
Analog signals from the detectors of the control systems are applied to the digital computer through a first multiplexer and an analog-to-digital converter. Digital signals from the computer are applied to the operating apparatus through a second multiplexer and digital-to-analog converters.
In the digital controller system there are back-up devices to back-up the digital computer upon occurrence of faults in the computer. The analog controller is generally employed as the backup device in the important control loop. These back-up devices are connected to the digital-to-analog converter, the detector and the operating apparatus, and are provided in the same numbers as the control systems. Switches of the first and second multiplexer and the analog-to-digital and digital-to-analog converters as the process input-output devices of the digital computer are required corresponding to the number of control systems. Moreover, in the conventional DDC system, wirings between the computer and the back-up device, for enabling both the computer and the back-up device to have the coincident set point variable, the communication wiring for displaying on whether the control is by the back-up device or by the computer and wirings for monitoring the adjustment output of the back-up device, etc. are required. Furthermore, in the conventional system, wirings for monitoring the process input variable and manipulated output variable and for the abnormal alarm must be connected to the respective display means separately and the system becomes much more complicated.
However, this digital controller system has the following defects or problems. More particularly, because back-up devices are required corresponding in number with control systems, wirings among the back-up devices, a central processor unit and control system become very complex. Another problem is that the numbers of process input-output devices increase correspondingly with the control systems. In addition, there arise problems such as the enlarged input-output section of each device and the complicated wiring job in the centralized control, as the result of the complication of wirings. And the problem of the weakened aggregation of the control system is also raised.
The present invention contemplates a new and improved computer system which eliminates all of the above preferred problems.